In regeneration of fluid catalytic cracking catalyst of the type used in modern conversion systems such catalyst is typically a combination of amorphous materials together with crystalline materials such as molecular sieves. Predominant components of such cracking catalysts are silica and alumina in a weight ratio of from about 10 to 60% alumina in the silica. Because alumina is aluminum oxide it is a highly abrasive material. Further, it is particularly abrasive because of its size. In such systems the catalyst normally has a particle size distribution of about 60-90 weight % in the range of 10-120 microns. Such dimensions are on the order of a finely ground flour or talcum powder. The particles flow and respond in a dry state very similar to a liquid.
After catalyst has been used to catalytically crack a hydrocarbon stream, the spent catalyst is coated with coke, primarily heavy hydrocarbon materials and carbon. This material is burned from the spent catalyst both to regenerate and heat it. The added heat is the primary driving energy required to crack the hydrocarbonaceous fluid and to circulate the catalyst. Depending upon the quality of the hydrocarbon feed stream, the catalyst is heated as high as it is reasonably economic to do so. The heated and regenerated catalyst generally flows by gravity from the regeneration bed to the riser reactor. Sufficient inert gas, such as steam, may be added to assist upward flow into a riser reactor. There the hot catalyst contacts a hydrocarbon stream. The actual driving force for the system is the voluminous evolution of hydrocracked gases upon hydrocarbon contact with the hot catalyst. The evolved gases are recovered as product vapors in a separator and the spent catalyst, with coke formed thereon, is returned to the regenerator, primarily by gravity, through a spent catalyst stripper.
In the regenerator the material is deposited on a grid and forms a bed through which air or other oxygen-containing gas is normally pumped under pressure to sustain the burning operation. The rate of flow of air through the bed is critical in that the air pressure also counterbalances the "hydrostatic" head of the "fluid" catalyst bed as it rests on the grid. The pressure through the air flow openings in the grid, either holes or screen, is maintained adequate to prevent catalyst from flowing back into the air supply, or a plenum chamber, normally formed below the bed, for equal distribution of air to catalyst over the entire grid.
Excess air flow through the regenerating catalyst can result in catalyst loss in the combustion gases; this results in overloading of the separating cyclones through which the combustion gases flow or the precipitator system for particles in the stack gases. More critically, excess air flow can decrease the temperature of the regenerated catalyst for reaction with hydrocarbons. Conversely, if air flow is inadequate, the particles will backflow into the air plenum chamber. Under this condition, such particles become mixed with air in the plenum and then re-enter the bed at high velocity through the same or other holes subjecting the air holes or openings to erosion. Such high velocity erosion can substantially increase the size of such airflow holes (either drilled in a plate or formed in a screen). If such holes or openings become too large it is difficult to maintain the plenum pressure high enough to counterbalance the hydraulic head of the catalyst bed. Under such circumstances, an FCC operation must be terminated and the grid repaired or replaced.
In accordance with the present invention, I have found that such problems of maintaining adequate air flow to regenerate the catalyst, but without excessive catalyst cooling or without catalyst backflow from the bed may be controlled by withdrawing a portion of the air in the regenerator plenum from directly adjacent a low point of the bed on the grid. The air withdrawn from the regenerator plenum may be either periodically sampled for catalyst particles by direct inspection or by measurement either directly or indirectly.
In one form of the invention such measurement may be made by directing the sampling air flow stream to the stack gas precipitator for the regenerator. The presence of catalyst particles in the stream may then be detected either by periodically sampling the air for particles or by measuring the temperature of gas flowing in the sample stream which would be elevated by the presence of hot catalyst therein. The rate of flow of the oxygen-containing gas to the plenum chamber, is then adjusted so that the plenum pressure is above that at which catalyst will backflow so that it can be detected in the sample stream.
In establishing such a level of air flow, the flow may first be reduced so that some catalyst is entrapped in the withdrawn air stream. Such catalyst is directly indicated by the presence of the particles or indirectly by measuring a temperature rise of the air stream. The flow is then raised by, say 10 percent, to assure no catalyst backflow. During continuous operation of the fluid catalytic system the backflow is then periodically checked to assure that the pressure required to maintain balance between the plenum air pressure and the head of fluid-like catalyst resting on the grid is adequate to prevent detection of hot particles in the sampled, or withdrawn, air stream.